For convenience of expression, the term "extract" is used herein to mean any soluble protein regardless of whether it might more appropriately be called an isolate or a liquor. Also, the term "chemical modification" is used herein to emcompass any treatment, such as hydrolysis, carboxylation, precipitation or additional separation, which occurs after the vegetable protein material is extracted and bleached.
Animal and vegetable proteins have been commercially used to make paper coating binders for many years. Casein, an animal protein, has historically played an important role in paper coatings. However, more recently, modified vegetable protein products, especially soy protein, have gained in commercial utilization. Today, modified soybean protein compositions are used as adhesive binders for piquant-containing coatings for paper and paperboard.
Clearly, it is desirable for paper coatings containing these soy protein and pigment compositions to have a bright white finish to contrast with printing inks. However, paper coatings prepared from these soy protein binders exhibit a reduced whiteness, as well as diminished gloss and brightness when compared with casein-containing coating binders.
Therefore, a bleach or oxidizing agent has sometimes been used to enhance the visual characteristics of the soy protein material. However, the process stage in which the soy protein material is treated with the bleach or oxidizing agent may vary depending upon the particular process being employed. For example, U.S. Pat. No. 2,246,466 discloses treating the alkaline extract of a protein with sodium peroxide. In contrast, U.S. Pat. No. 2,274,983 discloses the preparation of protein adhesives through hydrolysis followed by oxidation, wherein the preferred oxidizing agent is hydrogen peroxide. Furthermore, U.S. Pat. No. 4,961,788 discloses treating a carboxylated adhesive protein with an oxidizing material, such as hydrogen peroxide.
Problems arise, however, when the soy protein material is hydrolyzed or carboxylated before it is oxidized. This process allows color bodies to form before the bleach is applied. These color bodies must subsequently be destroyed by using substantial quantities of an oxidizing agent. These problems may be minimized by increasing the effectiveness of the oxidizing agent. However, these problems can be eliminated altogether and the visual characteristics of the final paper coatings can be enhanced if the soy protein material is oxidized with an effective oxidizing agent before any further chemical modification of the protein is performed.
Bleaching technology in the pulp and paper making industry uses peroxides in conjunction with various metal salts, such as magnesium salts. U.S. Pat. Nos. 4,132,589, 4,626,319, 4,859,283 and 4,915,785 disclose the bleaching of pulp through the use of peroxide and magnesium salts.
An object of the invention is to provide new and improved methods of producing modified vegetable protein binders for paper coatings.